Railroad hopper cars carrying bulk particulate matter, such as grain or plastic pellets, are typically unloaded by applying a vacuum conveying line to an outlet gate positioned at the bottom of each car compartment. The primary air flow for the vacuum conveying line is obtained from the exterior of the car. That is, the exhaust air for unloading is not drawn from the car compartment through the particulate matter. Nevertheless, the top of the car compartment must be vented to compensate for material drawn out the bottom of the compartment. Failure to vent the top of the compartment would reduce the efficiency of the vacuum unloading process and even risk damage to the car structure. Certain cars have been equipped with vents, but these are inadequate for unloading purposes. Brown, U.S. Pat. No. 2,324,356 is an example. Most commonly, venting for unloading of hopper cars has been achieved by propping open at least one hatch cover on each hopper compartment being unloaded.
Opening hatch covers presents disadvantages. First, some means permitting safe access to the top of the railroad car must be provided. One possibility is to attach ladders and platforms to the car which a worker can use to climb onto the top of the car. Or the unloading facility may have a gantry or the like, providing safe access to the top of the railroad car. Either of these methods unduly complicates the necessary facilities. Secondly, open hatches tend to invite security problems. Not only is there a risk of contaminating the lading due to the open hatch, but also empty compartments are tempting disposal sites for assorted refuse after the lading has been removed. Further, unless the hatch cover is secured by hinges or the like, there is a possibility of the cover being misplaced.
A vented hatch cover for use in the interior of a car is taught by Talmey, U.S. Pat. No. 2,641,202. In essence, Talmey has a double-walled roof structure with a sealed hatch door on the outer wall and a vent in a hatch cover in the inner wall which overlies the hatch opening. If the roof hatch is open, any contaminant such as rain, snow, or the like, may enter the associated vessel directly through the filter. If the roof hatch is not open, only air, available between the walls of the car, can enter the compartment through the inner cover but exterior air is not available.